Category: Claims handling · Reviewed by Chrissie Anderson, Client Executive · Last reviewed 2026-06-11
A series clause is an aggregation clause that treats multiple claims arising from a series of related acts, omissions, matters or transactions as a single claim for the purposes of applying the policy limit and deductible.
The series clause is the dominant aggregating wording in professional indemnity insurance, particularly under the SRA Minimum Terms, the ICAEW PI standard wording, the RICS minimum terms and most modern bespoke PI policies. It addresses the common scenario where a single underlying error or course of conduct produces multiple separate claims — different clients, different transactions, different damages — but where the underlying cause is one and the same.
The clause typically reads: “Claims arising from one act or omission; one series of related acts or omissions; the same act or omission in a series of related matters or transactions; or similar acts or omissions in a series of related matters or transactions where there is a common originating cause, shall be regarded as one claim.”
Application of the clause is intensely fact-sensitive. The most important English case on its operation is AIG Europe Ltd v OC320301 LLP (Woodman) [2017] UKSC 18, which held that “related” in the SRA wording requires the matters or transactions to be connected to each other in some dependent way, not merely similar.
The principal authority is Woodman. The Supreme Court held that the SRA Minimum Terms aggregation language requires the matters or transactions in question to be related to each other through some dependent factual link — that they “fit together” in a meaningful way. Mere similarity of subject matter is not enough.
The decision unsettled the prior assumption (drawn from Lloyds TSB v Lloyd’s Underwriters and other authorities) that a single underlying cause was sufficient to aggregate. Woodman did not overturn that line; it refined it. Where the wording requires a “series” of related matters, the matters themselves must be related — through a common transaction, common subject matter or some other dependent feature — not just share a common cause.
Prior authority continues to be relevant. Countrywide v Marshall [2002] EWHC 2082 addressed financial-services mis-selling. Spire Healthcare Ltd v Royal & Sun Alliance Insurance plc [2022] EWCA Civ 17 — concerning a wording on “one source or original cause” in the medical-malpractice context — reinforced that aggregation language must be read carefully to identify the operative aggregating concept.
The Insurance Act 2015 does not modify aggregation directly but its general principles on construction apply. Where the wording is genuinely ambiguous, contra proferentem construction (against the drafter) is the long-standing canon, though its modern application is more measured.
A series clause analysis runs through five steps. First, identify the alleged “series”: which claims are being grouped, and on what basis? Second, identify the unifying factor: is it a common act, a common omission, a common transaction or a common cause? Third, check the wording: which limb of the clause is engaged? Fourth, test the connection: do the relevant matters relate to each other in a dependent way under Woodman, or are they merely similar? Fifth, conclude: do the claims aggregate or not?
The analysis is often counter-intuitive. Two claims arising from the same negligent advice given to the same client on the same day clearly aggregate. Two claims arising from the same defective conveyancing precedent used for different clients on different transactions may or may not aggregate, depending on how closely the transactions are otherwise connected.
The factual matrix matters more than the abstract legal test. Examples drawn from the case law:
The implications cascade into reserving, recovery, defence strategy and reinsurance. An insurer who concludes early that a series of claims aggregates will treat the cluster as a single claim, applying one deductible and reserving toward one limit. An insurer who concludes the opposite will reserve and handle each claim separately, with consequent administrative cost but greater aggregate cover available.
“Sweeper” series clauses use very broad language (“arising from any one cause, occurrence or series of causes or occurrences”) to capture as much as possible. These are typically narrower in practice than the words suggest because of how courts construe them.
“Per-client” variations limit aggregation to claims by a single client, avoiding cross-client aggregation. Useful in financial-services contexts.
“Per-transaction” variations limit aggregation to claims arising from a single transaction or matter, regardless of the act or omission’s similarity to others.
“No-series” wordings exclude series aggregation, treating each claim separately. Rare in modern PI cover but found in some bespoke wordings.
An accountancy firm provided audit services to fifteen subsidiaries of a single holding company group over three financial years. A single audit-methodology error (failure to test inventory adequately) led to misstatements in the accounts of nine of the subsidiaries, which the holding company group then relied on in a sale-and-purchase transaction. Nine separate claims followed, totalling £18m in aggregate damages. The accountancy firm’s PI policy has £5m per claim and £15m aggregate cover with a series clause modelled on the SRA wording. Aggregation analysis: the audits were of related entities within a single group, the engagements were sequential and connected through the group relationship, the same methodology error produced the same kind of misstatement. The series of audits is a “series of related matters” and the claims aggregate. One £5m limit; £13m of the loss is uninsured.
By Matt Bartlett, Director, on 2026-06-11. Next review: 2026-12-11.
This entry is part of the Apex Insurance Wiki. Last reviewed by Matt Bartlett on 2026-06-11. Apex Insurance Brokers Limited, FCA FRN 724952, Companies House 07014570. Not regulated advice — consult your broker on your specific position.
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